1. Field of the Invention
This invention relates to organosiloxane compositions. More particularly, this invention relates to a method for extending the working life at room temperature of organosiloxane compositions curable by a platinum-catalyzed hydrosilation reaction without adversely affecting the cure rate of these compositions at temperatures of about 100.degree. C. and above.
2. Description of the Prior Art
One mechanism for curing organosiloxane compositions is by a hydrosilation reaction between a polyorganosiloxane containing two vinyl or other ethylenically unsaturated hydrocarbon radicals per molecule and an organohydrogensiloxane containing an average of at least three silicon bonded hydrogen atoms per molecule. Alternatively each molecule of the polyorganosiloxane contains three or more vinyl radicals and the organohydrogensiloxane contains two silicon bonded hydrogen atoms per molecule. The reaction is conducted in the presence of a platinum compound that is at least partially soluble in the reaction mixture and catalyzes the hydrosilation reaction.
In some instances the curing reaction will begin at temperatures of about 25.degree. C. once all of the reactants and catalyst have been combined. For some applications it is desirable to delay curing of the composition at these temperatures in order to store the composition for several hours prior to curing to obtain additional working time and/or ensure that the composition will completely fill all parts of a mold before curing to the extent that the composition is no longer flowable.
Many different classes of compounds will inhibit platinum catalyzed hydrosilation reactions. One type of inhibitor is volatile and must be removed from the composition by evaporation for the hydrosilation reaction to proceed. Non-volatile inhibitors are inactivated by heating.
Volatile inhibitors include the vinyl-containing cyclic organosiloxanes disclosed in U.S. Pat. No. 3,923,705, which issued to Smith on Dec. 2, 1975, the acetylenic alcohols such as 2-methyl-3-butyn-3-ol described in U.S. Pat. No. 3,445,420 that issued to Kookootsedes et. al on May 20, 1969, and some of the heterocyclic amines such as pyridine disclosed in U.S. Pat. No. 3,188,299 that issued on Jun. 8, 1965.
Non-volatile inhibitors are exemplified by the alkyl maleates disclosed in U.S. Pat. No. 4,256,870, which issued to Ekberg on Mar. 17, 1981, and the olefinic siloxanes described in U.S. Pat. No. 3,989,667, which issued to Lee and Marko on Nov. 2, 1976.
The olefinic siloxanes described in the aforementioned Lee and Marko patent are reaction products of secondary or tertiary acetylenic alcohols with siloxanes having silicon-bonded hydrogen atoms. The reaction products contain from 3 to 10 siloxane units with a total of at least three units of the formula RHSiO or R.sub.2 HSiO.sub.0.5 and at least one unit of the formula ##STR1## where R represents a monovalent radical selected from the group consisting of hydrocarbon radicals and perfluoroalkylethylene radicals and R' represents a monovalent hydrocarbon radical substituted with a secondary or tertiary hydroxyl group.
Because the reaction products inhibit the hydrosilation reaction used to prepare them, the mixture of acetylenic alcohol, siloxane and platinum catalyst must be heated at temperatures above 100.degree. C. to inactivate the inhibitor. In accordance with the teaching of Lee and Marko, the mixture of reactants is passed through a tube wherein the mixture is heated above 100.degree. C. under sufficient pressure to maintain the reactants in the liquid state. The reaction product is typically a mixture wherein one or more of the silicon bonded hydrogen atoms on each molecule of the initial organohydrogensiloxane have been reacted.
The inhibitors of Lee and Marko are less than desirable because of the high temperatures and pressures required to prepare them and the difficulty of obtaining a pure material that will function reproducibly as an inhibitor.
U.S. Pat. No. 3,699,073 which issued to Wada et al. on Oct. 17, 1972, hereinafter referred to as the Wada patent, discloses room temperature curable organopolysiloxane compositions comprising a diorganopolysiloxane containing a silicon-bonded vinyl radical at the two terminal positions of each molecule (identified in the patent as "component 1"), an organohydrogenpolysiloxane, an inorganic filler, a platinum-containing catalyst to promote curing of the composition and an optional copolymer consisting of SiO.sub.2, triorganosiloxy and diorganovinylsiloxy units, where the silicon bonded organic groups are monovalent hydrocarbon radicals that are free of ethylenic unsaturation. The inventive ingredient of these compositions is an organosiloxane copolymer, referred to in the patent as "component 3", wherein the main chain is composed of RR'SiO units (a) or a mixture of these units and R'SiO.sub.3/2 units (b) and the copolymer is terminated with R"R.sub.2 SiO.sub.1/2, R"OSi(R'.sub.2)O.sub.0.5 or HOSi(R'.sub.2)O units. The molar ratio of b units to the total of a and b units does not exceed 0.5. In these formulae R represents a monovalent hydrocarbon radical that is free of ethylenic unsaturation, at least one percent of the radicals represented by R' are vinyl with the remainder selected from the group as R, and R" represents a saturated or ethylenically unsaturated hydrocarbon radical.
Example 1 of the Wada patent demonstrates that the presence of 3 parts by weight of an organosiloxane copolymer of the type referred to hereinabove as component 3 per 100 parts of vinyl-terminated diorganopolysiloxane enabled the composition to remain pourable for about 6 hours. The composition cured in 24 hours under ambient conditions.
For some end-use applications it is desirable to achieve room temperature storage and/or working times longer than 6 hours for organosiloxane compositions of the type described in the aforementioned Wada et al. patent using the same amount of inhibitor.
The Wada patent does not teach a method for preparing the copolymer referred to as component 3. The only discussion of preparative methods appears in connection with the vinyl-terminated diorganopolysiloxane identified as "component 1". These methods, described in column 3 beginning at line 11, include "cohydrolyzing corresponding chlorosilanes or alkoxysilanes, or by subjecting divinyltetraorganodisiloxane and diorganosiloxane to an equilibrium reaction. The term "diorganosiloxane" can refer either to linear or cyclic diorganosiloxanes. No mention is made of the type of catalyst used for this reaction, leading to the assumption that the choice of catalyst is not critical. The catalysts typically used for this type of reaction are acids such as the alkylsulfonic acids and bases such as alkali metal salts, hydroxides and silanolates.